Relatively pure oxygen (i.e. an oxygen-containing gas having an oxygen content of 88% or more) has a number of desirable industrial and medicinal applications at various pressures and purities. The Earth's atmosphere, typically comprising nearly twenty one percent oxygen gas, is the natural candidate for use as an economical oxygen source. As a result, many of the most practical and economical oxygen production plants employ air separation systems and methods.
One of the more common systems for producing oxygen in relatively large volumes incorporates cryogenic technology to liquefy and separate a desired oxygen component of a predetermined purity from the air mixture. While this design works well for high-volume oxygen production (more than 300 tons per day) and/or very high-purity oxygen (such as 97-99.99%) the specialized cryogenic hardware and associated high capital start-up expenditures make such systems costprohibitive when used for production in low to moderate volumes e.g. from about 30 to about 200 tons per day of an oxygen containing gas with an oxygen concentration higher than about 88% and up to about 95%.
For manufacturing oxygen in moderate scale quantities and relatively low purities (typically 25 to 40%), a practical and highly advantageous air separation system utilizes a polymer membrane. The membrane has a relatively high selectivity to oxygen and high flux for nitrogen. Compressed air feeds the membrane that retains the desired oxygen component at a relatively moderate purity and passes the remaining (undesirable) components as waste. The oxygen content of the retentate can be increased by provision of two or more successive membrane separation stages. As an alternative to both multi-stage membrane technology and cryogenic processes, those skilled in the art have developed an air separation system that utilizes a molecular sieve adsorbent to efficiently produce oxygen at high purities usually ranging from approximately 88 to 93% and up to about 95%. Used in pressure swing adsorbent (PSA) and vacuum pressure swing adsorbent (VPSA) systems, the adsorbent typically acts on the quadrupole moment between the respective gas components (nitrogen and oxygen) in air to effect component separation.
The original PSA process was developed by Skarstrom, U.S. Pat. No. 2,944,627, and consists of a cycle including four basic steps: (1) Adsorption, (2) Depressurization, (3) Purge, and (4) Repressurization. Several variations of the Skarstrom cycle have evolved. One such system is described in Wagner U.S. Pat. No. 3,430,418, wherein at least four beds are required to produce product continuously. The extra cost and complexity of providing four beds rather than a lesser number (preferably two) generally makes the Wagner system economically unfeasible.
In U.S. Pat. No. 3,636,679, Batta described a system where compressed air and product oxygen (obtained from another bed going through the equalization falling step) are simultaneously introduced at opposite ends of the same adsorbent bed. Another process for achieving further savings in equipment cost by using a two bed system is described by McCombs in U.S. Pat. No. 3,738,087, wherein an increasing pressure adsorption step is employed with feed air introduced to a partially repressurized adsorbent bed. Following the work of McCombs, Eteve et al., U.S. Pat. No. 5,223,004 described a PSA process utilizing the following steps: (1) a countercurrent product pressurization starting from the low pressure level of the cycle to an intermediate pressure level, (2) a cocurrent feed pressurization from the intermediate pressure level up to the adsorption pressure without bleeding off, (3) a production step wherein air is admitted and oxygen is bled off cocurrently, (4) a step where oxygen is bled off by partial depressurization cocurrently, wherein the admission of air is discontinued, and (5) a desorption step by depressurization countercurrently down to the low pressure level of the cycle.
Many more variations of the original PSA cycle can be found in the literature. For example, U.S. Pat. Nos. 4,194,891, 4,194,892 and 5,122,164 describe PSA cycles using short cycle times, wherein smaller particle size adsorbents are used to reduce diffusive resistance; Doshi et al, U.S. Pat. No. 4,340,398, discloses a PSA process utilizing three or more beds, wherein void gas is transferred to a tank prior to bed regeneration, and later used for repressurization. In addition, a process modification to a two-bed PSA process incorporating tank equalization is disclosed in U.S. Pat. Nos. 3,788,036 and 3,142,547, where the conserved gas is used as the purge gas for another bed.
More recently, Tagawa et al., U.S. Pat. No. 4,781,735, discloses a PSA process using three adsorbent beds to produce oxygen, with enhanced oxygen recovery achieved by connecting the feed end of one bed to the feed end of another bed (bottombottom equalization), and for all or part of the equalization time, top-top bed equalization is carried out simultaneously with the bottom-bottom equalization. In addition, U.S. Pat. No. 5,328,503, Kumar et al, describes a PSA process that uses an initial depressurization step to provide a purge gas, followed by an optional bed-bed pressured equalization step. In accordance with this patent, at least two adsorbent beds are employed, and a combination of product and feed gas are used for repressurization of the adsorbent beds.
Liow and Kenny (AICHE J. vol. 36, p. 53, 1990) disclose a "backfill cycle" for oxygen production from air via computer simulation. They disclose that a countercurrent (with respect to feed direction) product repressurization step is beneficial when included in the cycle for producing an enriched oxygen product.
In U.S. Pat. No. 5,518,526 of Baksh et al, an improved PSA process is disclosed for separating a first gas, such as an oxygen containing gas, from gas mixtures such as air. The process involves the steps of simultaneous equalization and evacuation followed by simultaneous feed and product gas repressurization of PSA beds. This results in an overall faster and more efficient cycle with 100% utilization of a vacuum (or pressure reducing) blower, and a reduction in power use of about 15% over previously known processes. More specifically, the Baksh et al. process involves overlapping of various steps of the PSA cycle to reduce total cycle time and thus improve productivity. Other important parameters include choice of operating conditions (the value of the high pressure, the value of the low pressure, the pressure at the end of equalization falling step, and the amount of high purity product used in the product pressurization step), the time period allocated for each step, the order in which each step of the cycle is executed, and the use of equalization falling gas to provide the gas required for refluxing and equalization rising. The cycle includes the step of evacuating one bed ("first bed") undergoing the equalization rising step while simultaneously the other bed ("second bed") is undergoing the equalization falling step. The time allocated for this step must be chosen, so that at the end of this step, the first bed has been purged and also partially pressurized. The next step in the cycle is simultaneous product and feed pressurization at opposite ends of the first bed, followed by feed pressurization to the desired adsorption pressure. Other features of the Baksh et al. process are as follows: (a) the product gas required in the step of simultaneous feed and product pressurization usually comes from the product tank, or from another bed in the production step; and (b) the cocurrent depressurization or pressure equalization falling gas either goes to the downstream end of another bed or to a second storage tank. In the latter case, no bed-bed communication is required, which adds further flexibility in controlling the PSA process.
Copending commonly assigned U.S. patent application Ser. No. 08/611,942 filed Mar. 7, 1996, now U.S. Pat. No. 5,702,504, is directed to a VPSA process similar to that of Baksh et al. and also comprising (a) an additional countercurrent depressurization step to the bottom (desorption) pressure during which step nitrogen rich gas is discharged from both the feed end (waste) and from the product end (used to repressurize another bed) interposed between (i) a cocurrent depressurization to an intermediate falling pressure (and collecting equalization gas for the other bed) and (ii) a subsequent countercurrent depressurization and evacuation of nitrogen; and (b) an additional discharge of relatively nitrogen rich gas from the feed end while simultaneously purging with oxygen after (ii) and before completing the discharge of gas from the feed end.
Despite such desirable advances in the art, PSA/VPSA processes remain less efficient and more capital intensive, than desired, especially for high purity (about 88% up to about 95%) oxygen production in large plants, particularly as compared to the alternative of cryogenic distillation. There is a need in the art, therefore, for further improvements to make the use of the highly desirable PSA/VPSA technology in commercial plants efficient and therefore more economical.
Modern conventional VPSA systems operated on a commercial scale typically include a feed gas compressor for feeding an air mixture to an adsorbent "bed" that includes the molecular sieve adsorbent. The bed operates to selectively adsorb nitrogen from the air mixture at a predetermined (upper) adsorption pressure. Oxygen, as the less readily adsorbed component of the mixture, passes through and is discharged from the bed as a product stream. Once the nitrogen is adsorbed by the surface of the adsorbent bed, a vacuum system is connected to the bed to reduce the pressure to a bottom (desorption) pressure, causing the adsorbed (nitrogen-rich) gas to desorb and to be discharged from the system as waste (or a byproduct). A purge mechanism plumbed to the bed cooperates with the vacuum system to purge residual nitrogen from the system. Oxygen is usually used as the purge gas.
A relatively efficient conventional industrial scale VPSA system having two or more adsorbent beds and associated separation method involves operating at relatively low pressure ratios (adsorption pressure/desorption pressure) typically approximating 4:1-5:1 with bottom (desorption) pressures 0.25-0.33 atmospheres or lower. Lower pressure ratios such as 2:1 or 3:1 in VPSA systems for the production of an oxygen rich gas containing 90% oxygen or higher have also been reported, e.g., in U.S. Pat. No. 5,074,892 to Leavitt. The system configuration used under such reduced pressure ratio operating conditions includes a conventional feed gas compressor, a multi-stage vacuum pump, and a pair of conventional adsorbent beds such as described generally above. The Leavitt patent acknowledges that bed size factor, i.e. the quantity of the adsorbent required to effect the separation, will increase with the reduced pressure ratio (although it will increase less than was expected based on the then state of the art). Leavitt does not propose any method for addressing the bed size factor increase nor any other method for further decreasing operating costs.
Thus, the prior art (both the patent/scientific literature and industrial practice) failed to realize that the lower pressure ratio alone or preferably together with certain process modifications would permit use of simplified and more economical equipment and more generally result in overall cost savings over and above those achieved with low pressure ratio alone. Generally, because conventional VPSA low pressure ratio systems having two adsorbent beds operate at relatively deep vacuums (even when the pressure ratio is relatively low), power-consuming multi-stage vacuum pumps (involving two or more vacuum stages) were typically required or were thought to be required. Such pumps often include two vacuum stages disposed in a cascade relationship with an interstage connection positioned therebetween. The interstage connection typically includes an inter-stage bypass unload system to discharge the suction of both stages to atmospheric pressure during idling or unloading operations. The bypass in turn includes an additional valve and associated plumbing to effect venting. Thus, multistage vacuum devices are considerably more complex than singlestage devices, and more expensive to operate.
The conventional two-bed VPSA systems also operate at longer cycle times (from a minimum of 40-50 seconds two 60-90 seconds for well-run, modern, conventional two-bed systems) in order to achieve the desired high oxygen recovery (usually within the range of about 40 to about 70%). This in turn increases not only the adsorbent requirement, but also the displacement of the entire system, contributing to the power consumption per unit product produced. Although it is possible to decrease cycle time somewhat, any advantage thus achieved is limited (or even eliminated) by the fact that higher gas velocities result, notably in the adsorber. Higher gas velocities cause the pressure drop across the adsorbent bed to increase, which lowers the efficiency of the process. Higher gas velocities also cause adsorbent "lifting" which then causes adsorbent attrition, another adverse contributor to overall cost. Thus, both of these consequences increase costs, which contravenes the purpose of using faster process cycles in the first place. Thus, with respect to two-bed, VPSA systems in the 30-200 tpd range that produce high-purity oxygen, and 90-95% in particular, there is still room for improvement of the process efficiency and for a further decrease in capital and/or operating costs. It should be noted that a savings in overall cost (operating cost and present value of capital cost) of even 1-2% is considered substantial in the air separation industry which is highly competitive.
Therefore, a need exists for a VPSA air separation system and method that implements a low pressure ratio and avoids the costly use of expensive vacuum equipment. Moreover, the need exists for lowering operating costs while still realizing attractive oxygen recoveries (e.g. 50-60% as opposed to 60-70%, generally representing a loss of 5-10 recovery points compared to a conventional two-bed system) to economically produce an oxygen containing gas. The vacuum pressure swing adsorption system and method of the present invention satisfies these needs. The present invention achieves lower overall cost through implementation of various novel process and apparatus features alone or in combination.